Ball sealers are small spheres having a resilient or compliant outer layer which are used in the oil and gas industry in connection with efforts to increase the production rate of wells through acid treatment or hydraulic fracturing.
Generally when ball sealers are used, an initial charge of treating fluid is first injected into the well. Then a number of ball sealers, which is less than the calculated total number of perforations in the casing of the well, is placed in the well. Next, additional treating fluid is injected under pressure and the ball sealers are carried by the flow of the fluid to those perforations in the well casing which are in the area of least resistance to flow. The ball sealers seat upon those perforations and divert the flow of the treating fluid to the remaining open perforations. The ball sealers are retained seated upon the perforations by the pressure differential of the treating fluid across the perforations. when the injection of treating fluid under pressure stops, the pressure differential across the perforations drops and the ball sealers become unseated.
The use of the ball sealers which divert the treating fluid from the area of least resistance to flow makes it possible for the treating fluid to reach areas of higher resistance which are normally untreated. The usual result is an increased production rate of hydrocarbons from the well.
In the Erbstoesser U.S. Pat. No. 4,102,401, a ball sealer is disclosed which has a syntactic foam inner core and an outer resilient layer of rubber or a similar elastomeric material. The Erbstoesser ball sealers are prepared by compressing an uncured rubber cover about a spherical inner core of syntactic foam in an arbor press and curing the rubber. Ball sealers prepared in this manner do not always have an outer layer of uniform and predictable thickness which is considered to be essential to insure good performance.
The ball sealer of the Erbstoesser patent has a density which is less than that of the treating fluid with which it is used. However, in the past, ball sealers have also been used which have a density which is higher than that of the treating fluids. The use of the low density sealers appears to be the more promising.
In order to be effective, ball sealers of either high or low density must have outer coatings or coverings which are sufficiently resilient or compliant to seat upon and to seal the jet formed perforations in the well casing. They must also have rigid inner cores which resist extrusion into or through the perforation. Otherwise, the ball sealers could penetrate into the pay zone and permanently damage the flow characteristics of the well. In addition, the ball sealers must be chemically inert in the environment in which they are used and resist the stresses caused by the hydrostatic pressure of the fluid in the well bore and the pumping pressures.